I have a Vokera Unica He boiler. About 10 years old. Everything works perfectly, heating, hot water, etc,, Just one little problem, when I run a bath, I get hot water for a minute or so, then it goes cold for a minute or so, then heats up again for a couple of minutes, then goes cold for a minute or so ... and so on. The thermostat seems to turn off, then on again. Bottom line is I cant have a bath. What is odd is that when I turn on the hot water at the sink, I get constant hot water for as long as I like. Anyone have any ideas? Gas man has been twice and due for third visit. No luck so far.

Cold bathwater

Hello and a warm welcome to the forum It is common for the domestic heat exchangers to get blocked on these. They block with small iron filings from within the central heating system. This is particularly noticeable more so when running bath/showers, as the boiler is trying to push all of its energy around a restricted circuit.Thanks